1. Field of the Invention
The present invention generally relates to projection optical systems as well as projection exposure apparatus for the purpose of reduction projection of patterns on the first object onto the second object such as a substrate, and particularly relates to projection optical systems as well as projection exposure apparatus which are ideal for projection alignment of integrated circuit patterns formed on a reticle (mask) as the first object onto a substrate (wafer) as the second object.
2. Related Background Art
As integrated circuit patterns become more refined, the performance required for projection optical systems used for wafer printing has become more rigorous. Under such a circumstance, in order to improve resolution of projection optical systems, it can be considered either to further shorten an exposure wavelength .lambda. or to increase a numerical aperture NA of projection optical systems.
In recent years, in order to accommodate further refinement of print patterns, a light source to be used for exposure has become substituted from the one that emits g line (436 nm) exposure wavelength to mainly the one that emits i line (365 nm) exposure wavelength. Furthermore, a light source that emits light with shorter wavelength, such as excimer laser (248 nm, 193 nm), is about to be used.
And a projection optical system which performs projection alignment of a pattern on a reticle onto a wafer by use of each of the aforementioned exposure wavelengths is being proposed.
What is required for a projection optical system along with improving resolution is reduction of image distortion. What is termed as "image distortion" herein are, besides the distortion due to a projection optical system (distorted aberration), the one due to curvature of a wafer to be imaged thereon on the image side of a projection optical system and the one due to curvature of a reticule on the object side of the projection optical system on which IC patterns and such are described.
Therefore, in order to lessen an influence due to wafer curvature upon image distortion, so-called an image side telecentric optical system, which places an exit pupil on the image side of the projection optical system farther, has been used.
Meanwhile, with regard to reduction of image distortion due to reticle curvature, it has been considered to make the object side of the projection optical system a telecentric optical system, and thus it has been proposed to place an entrance pupil position of the projection optical system comparatively farther from the object. Some of such examples for are Patent Disclosure Showa #63-118115, Patent Disclosure Heisei #4-157412, and Patent Disclosure Heisei #5-173065.
The projection optical systems proposed in each of the above mentioned Patent public information, however, have small numerical apertures NA and do not have high resolution, or have narrow exposure areas. Also with regard to an entrance pupil position of the projection optical systems, it was simply placed comparatively farther from the object plane (reticle), and telecentricity on the object side (reticle side) and on the image side (wafer side) was not corrected adequately.
Also, although some of the projection optical systems proposed in the aforementioned Patent Information materialized telecentricity on the object side by increasing the distance from the object plane (reticle) to a first lens surface of the projection optical system, since the distance between the object and the image (the distance between the reticle and the wafer) has become extremely long compared to the exposure area, it has brought such a problem as growing of the size of the optical system per se.
The present invention was conducted in light of the above mentioned issues, and its objective is, while maintaining a comparatively large exposure area, to offer a compact projection optical system which has high-resolution and a large numerical aperture which obtains substantially small image distortion even in the case where the flatness of the first object (reticle) as well as the second object (wafer) is extremely bad, and to offer a projection exposure provided therewith.